As reactions which are carried out in the presence of a catalyst at a high temperature, there have been known treatment of an organic solvent or deodorization by oxidiation, treatment of automobile exhaust gas, high-temperature steam reforming and high-temperature denitration. Recently, a catalytic combustion technique is employed in large-volume boilers, gas turbines and aircraft gas turbines.
In these treatment processes, a reaction temperature of at least 600.degree. C. is employed and under some conditions, a temperature of as high as 1400.degree. to 1500.degree. C. is employed. Therefore, catalysts having a high thermal stability the activity of which is not seriously damaged at such a high temperature are demanded.
As high-temperature catalysts, there have been used those comprising a noble metal or a base metal supported on a carrier comprising alumina, silica or silica/alumina; or those comprising a noble metal component supported on a carrier comprising a ceramic material such as zirconia, aluminum titanate or silicon nitride coated with active alumina or the like.
However, these catalysts have defects that when they are heated to a temperature of generally above 800.degree. C., the specific surface area thereof is reduced due to phase transition of the carrier or crystal growth and also the surface area is reduced due to coagulation of the active component to deteriorate the catalytic activity thereof remarkably. The catalysts made of the ceramic material have a defect that the catalytic component cannot be used effectively, since the heat resistance of the coating material is low.
There have been known catalysts wherein carrier materials are made of Al.sub.2 O.sub.3 : and lanthanoides as disclosed in the following U.S. Patents.
(1) U.S. Pat. No. 3,993,572 "Rare Earth and Platinum Group Metal Catalyst Composition" PA0 (2) U.S. Pat. No. 3,966,391 "Method of Combustion using High Temperature Stable Catalysts" PA0 (3) U.S. Pat. No. 3,956,188 "Compositions and Methods for High Temperature Stable Catalysts" PA0 (4) U.S. Pat. No. 3,899,444 "Exhaust Gas Catalyst Support" PA0 (5) U.S. Pat. No. 3,867,312 "Exhaust Gas Catalyst Support" PA0 (6) U.S. Pat. No. 3,714,071 "Low Density Alumina Spheres of Improved Strength at High Temperature" PA0 (7) U.S. Pat. No. 4,056,489 "Hih Temperature Stable Catalyst Composition and Method for its Preparation" PA0 (8) U.S. Pat. No. 4,021,185 "Compositions and Methods for High Temperature Stable Catalyst" PA0 (9) U.S. Pat. No. 4,220,559 "High Temperature-Stable Catalyst Composition" PA0 (10) U.S. Pat. No. 4,061,594 "Alumina-based Bodies Obtained by Agglomeration which are resistant to Elevated Temperature"
Other prior arts include U.S. Pat. Nos. 3,978,004, 3,956,186, 3,931,050, 3,898,183, 3,894,140, 3,883,445, 3,880,775, 3,867,309, 3,819,536, 4,374,819, 4,369,130, 4,318,894, 4,233,180, 4,206,087, 4,177,163, 4,153,580, 4,170,573, and 4,054,642.
Among the prior art listed above, U.S. Pat. Nos. 3,966,391, 4,170,573 and 4,061,594 seem to be relevant to the present invention. The U.S. Pat. No. '391 discloses a process for producing a catalyst which comprises preparing a solution containing La(NO.sub.3).sub.3, CrO.sub.3 and Sr(NO.sub.3).sub.3, adding Al.sub.2 O.sub.3 powder in the solution, stirring the mixture to impregnate the Al.sub.2 O.sub.2 powder with the solution, heating the mixture under stirring to evaporate the liquid, drying the mixture at 110.degree. C., and calcining the dried mixture at 1200.degree. C. for 2 hr. The catalysts obtained are used for combustion of carbonaceous fuel.
The U.S. Pat. No. '573 discloses a process for preparing a catalyst which comprises impregnating Al.sub.2 O.sub.3 powder with an La(NO.sub.3).sub.3 solution, drying the impregnated Al.sub.2 O.sub.3 powder at 160.degree. C. for 16 hrs, calcining the dried Al.sub.2 O.sub.3 powder at 1250.degree. C. for 1 hr, impregnating the calcined Al.sub.2 O.sub.3 powder with a Ce(NO.sub.3).sub.3 solution, drying the impregnated La-Ce-Al.sub.2 O.sub.3 powder at 160.degree. C. for 16 hrs, impregnating the La-Ce-Al.sub.2 O.sub.3 powder with a PtCl.sub.4 solution, and calcining the Pt-La-Ce-Al.sub.2 O.sub.3 powder at 427.degree. to 649.degree. C.
The U.S. Pat. No. '594 discloses a process for producing a catalyst which comprises autoclaving Al.sub.2 O.sub.3 at 600.degree. C., calcining the precipitate at 500.degree. C., impregnating the calcined with a La(NO.sub.3).sub.3 solution, drying the impregnated Al.sub.2 O.sub.3, impregnating the dried La.sub.2 O.sub.3 -Al.sub.2 O.sub.3 with a solution of a platinum group element, and calcining the impregnated La.sub.2 O.sub.3 -Al.sub.2 O.sub.3 at 1000.degree. C. or 1200.degree. C.
Journal of Solid State Chemistry 19, 193-204 (1976) discloses structural investigation of the lanthanum .beta.-Alumina phase, the disclosure of which is not concerned with a catalyst.